February 17, 2004: NASA has a mystery to solve: Can people
go to Mars, or not?

"It's a question of radiation," says Frank Cucinotta of
NASA's Space Radiation Health Project at the Johnson Space Center.
"We know how much radiation is out there, waiting for us between
Earth and Mars, but we're not sure how the human body is going to
react to it."

NASA astronauts have been in space, off and on, for 45 years. Except
for a few quick trips to the moon, though, they've never spent much
time far from Earth. Deep space is filled with protons from solar
flares, gamma rays from newborn black holes, and cosmic rays from
exploding stars. A long voyage to Mars, with no big planet nearby
to block or deflect that radiation, is going to be a new adventure.

NASA weighs radiation danger in units of cancer risk. A healthy 40-year-old
non-smoking American male stands a (whopping) 20% chance of eventually
dying from cancer. That's if he stays on Earth. If he travels to Mars,
the risk goes up.

The question is, how much?

"We're not sure," says Cucinotta. According to a 2001 study
of people exposed to large doses of radiation--e.g., Hiroshima
atomic bomb survivors and, ironically, cancer patients who have undergone
radiation therapy--the added risk of a 1000-day Mars mission lies
somewhere between 1% and 19%. "The most likely answer is 3.4%,"
says Cucinotta, "but the error bars are wide."

The odds are even worse for women, he adds. "Because of breasts
and ovaries, the risk to female astronauts is nearly double the risk
to males."

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Researchers who did the study assumed the Mars-ship would be built
"mostly of aluminum, like an old Apollo command module,"
says Cucinotta. The spaceship's skin would absorb about half the radiation
hitting it.

"If the extra risk is only a few percentâŚ we're OK. We
could build a spaceship using aluminum and head for Mars." (Aluminum
is a favorite material for spaceship construction, because it's lightweight,
strong, and familiar to engineers from long decades of use in the
aerospace industry.)

The error bars are large, says Cucinotta, for good reason. Space
radiation is a unique mix of gamma-rays, high-energy protons and cosmic
rays. Atomic bomb blasts and cancer treatments, the basis of many
studies, are no substitute for the "real thing."

The
greatest threat to astronauts en route to Mars is galactic cosmic
rays--or "GCRs" for short. These are particles accelerated
to almost light speed by distant supernova explosions. The most dangerous
GCRs are heavy ionized nuclei such as Fe+26. "They're
much more energetic (millions of MeV)
than typical protons accelerated by solar flares (tens to hundreds
of MeV),"
notes Cucinotta. GCRs barrel through the skin of spaceships and people
like tiny cannon balls, breaking the strands of DNA molecules, damaging
genes and killing cells.

Astronauts have rarely experienced a full dose of these deep space
GCRs. Consider the International Space Station (ISS): it orbits only
400 km above Earth's surface. The body of our planet, looming large,
intercepts about one-third of GCRs before they reach the ISS. Another
third is deflected by Earth's magnetic field. Space shuttle astronauts
enjoy similar reductions.

Apollo astronauts traveling to the moon absorbed higher doses--about
3 times the ISS level--but only for a few days during the Earth-moon
cruise. GCRs may have damaged their eyes, notes Cucinotta. On the
way to the moon, Apollo crews reported seeing cosmic ray flashes in
their retinas, and now, many years later, some of them have developed
cataracts. Otherwise they don't seem to have suffered much. "A
few days 'out there' is probably safe," concludes Cucinotta.

Right:
Apollo command modules were well-enough shielded for quick trips to
the Moon and back. [More]

But astronauts
traveling to Mars will be "out there" for a year or more.
"We can't yet estimate, reliably, what cosmic rays will do to
us when we're exposed for so long," he says.

Finding out is the mission of NASA's new Space Radiation Laboratory
(NSRL), located at the US Department of Energy's Brookhaven National
Laboratory in New York. It opened in October 2003. "At the NSRL
we have particle accelerators that can simulate cosmic rays,"
explains Cucinotta. Researchers expose mammalian cells and tissues
to the particle beams, and then scrutinize the damage. "The goal
is to reduce the uncertainty in our risk estimates to only a few percent
by the year 2015."

Once the risks are known, NASA can decide what kind of spaceship
to build. It's possible that ordinary building materials like aluminum
are good enough. If not, "we've already identified some alternatives,"
he says.

How
about a spaceship made of plastic?

"Plastics
are rich in hydrogen--an element that does a good job absorbing cosmic
rays," explains Cucinotta. For instance, polyethylene, the same
material garbage bags are made of, absorbs 20% more cosmic rays than
aluminum. A form of reinforced polyethylene developed at the Marshall
Space Flight Center is 10 times stronger than aluminum, and lighter,
too. This could become a material of choice for spaceship building,
if it can be made cheaply enough. "Even if we don't build the
whole spacecraft from plastic," notes Cucinotta, "we could
still use it to shield key areas like crew quarters." Indeed,
this is already done onboard the ISS.

If plastic isn't good enough then pure hydrogen might be required.
Pound for pound, liquid hydrogen blocks cosmic rays 2.5 times better
than aluminum does. Some advanced spacecraft designs call for big
tanks of liquid hydrogen fuel, so "we could protect the crew
from radiation by wrapping the fuel tank around their living space,"
speculates Cucinotta.

Can people go to Mars? Cucinotta believes so. But first, "we've
got to figure out how much radiation our bodies can handle and what
kind of spaceship we need to build." In labs around the country,
the work has already begun.

Stay tuned to Science@NASA in the weeks ahead for more installments
in our continuing series about space radiation. Next up: "Watch
out for Solar Flares."